The present invention relates to a method and a structure for protecting a work piece in a semiconductor manufacturing process, and more particularly to a method and a structure for protecting a wafer in an etching process.
In a semiconductor manufacturing process, gaseous hydrofluoric acid (HF vapor) is usually used for removing the remaining polymer on a silicon surface after a reactive ion etching (RIE) process. However, it can also etch away the underlying oxide layer which is deposited for protecting the silicon surface from being damaged in the subsequent implantation, and affect the yield rate of the wafers.
There are many kinds of electronic circuits distributed on a silicon surface in an electric device. Take a transistor for example, an oxide layer (12) is formed on the silicon surface (16) as shown in FIG. 1. After executing a RIE process, a polymeric residue (15) may be remained on the top and the edge of the oxide layer (12). The HF vapor (17) is used to remove this polymeric residue (15).
Referring to FIG. 2, the wafers are collected in a cassette (11). This cassette has 25 slots wherein 24 wafers are mounted thereon from the slot 1 to the slot 24 respectively, but nothing is mounted in the slot 25.
The cassette (11) is placed on an etching machine (21) when applying the HF vapor to remove the polymeric residue as shown in FIG. 3. The etching machine (21) is a single-wafer processing machine. Two areas are located on the working platform of the etching machine, one is a loader area (22) and the other is an unloader area (23). The cassette (11) having 24 wafers is placed on the loader area (22). The etching machine 21 picks a wafer from the cassette on the loader area at a time and then transfer the processed wafer to the unloader area until 24 wafers are processed.
Please refer to FIG. 4, which is a curve illustrating the gate oxide loss of the wafer where the x-axis specifies the number of the slots and the y-axis specifies the gate oxide loss of the wafer. According to these experimental data, the gate oxide loss of the wafer from slot 1 to slot 23 are not to much, but the oxide loss of the wafer in slot 24 has been increased dramatically. The increased oxide loss means that the oxide layer on a silicon surface becomes much thinner which will influence the following steps of the manufacturing process, for example, the silicon surface will be damaged when executing an ion implantation, thereby reducing the device relability.
Therefore, a major object of the present invention is to provide a process which can improve the problem of large gate oxide loss of the wafer, and avoid bad influence in the following processes.
Another object of the present invention is to provide a process which can improve the yield rate of the wafer in order to improve the quality of these semiconductor devices.
The present invention provides a method for protecting a wafer in an etching process. The method includes steps of providing a cassette for mounting therein the wafers, and providing a sheet piece for shielding the wafers in the cassette.
There are 25 slots for respectively mounting wafers and the sheet piece in this cassette. 24 wafers are mounted from slot 1 to slot 24 of the 25 slots and a sheet piece is mounted in slot 25. This sheet piece is a dummy wafer.
According to the present invention, the etching process is a gaseous hydrofluoric acid etching process for removing the polymeric residue on the silicon surface of the wafer. The etching process is carried out on an etching working platform mounted on an etching machine and is to etch a wafer in the slot 1 first. The etching process is finished after the wafer in the slot 24 is etched.
The working platform has a lid covering the working platform in order to prevent a contaminant, e.g. moisture, from entering the cassette on the working platform. The etching machine is a single-wafer processing machine which etches one wafer at a time.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: